Significance: Anterior cruciate ligament (ACL) tears are among the most common sport-related injuries in young athletes, especially females. Anterior cruciate ligament reconstruction (ACLR) surgery is the preferred treatment, yet the re-injury rate is unacceptably high in young athletes who return to sport (RTS) after ACLR. ACL injuries and re-injuries often occur during dynamic foot-ground interactions (e.g., cutting or landing). Deficient neuromus- cular control has been identified as the primary modifiable risk factor contributing to these injuries and re-injuries. Thus, recovery of neuromuscular control of dynamic foot-ground interactions should be a central criterion for safe RTS clearance, but is currently difficult to assess in typical clinical settings. Clinical need: Typical clinical settings lack technology to safely, economically and efficiently quantify recovery of neuromuscular control after ACLR to (i) improve RTS decision-making and (ii) avoid premature RTS. As a result, clearance criteria and success with RTS varies widely across clinical centers. Thus, there is need for a clinically practical means to quantify the recovery of neuromuscular control for RTS after ACLR. Scientific premise: The leg dexterity test safely and quickly quantifies neuromuscular control, correlates with dynamic full weight-bearing athletic agility, and captures sex differences. Thus, it has clear potential as a clinically practical means to quantify recovery of neuromuscular control for RTS decision-making. Rigor: Neuromuscular Dynamics, LLC commercializes simple, quick and safe patented technology shown by 20+ publications to quantify neuromuscular control. The lower-extremity version (i.e., leg dexterity test, Right) consists of asking seated participants to use a single leg to compress an unstable platform atop a slender spring. The spring buckles with low forces at ~15% of body weight, so the greatest mean force subjects can sustain quantifies their neuromuscular control of unstable foot-ground interactions, without exposure to high forces or risky weight-bearing maneuvers. Due to its scientific and technical merit, simplicity, and test/retest reliability, the leg dexterity test has clear potential as a clinical tool to quantify neuromuscular control in the outpatient setting. Goal: Establish the clinical feasibility and utility of using the leg dexterity test as an objective measure of recovery of neuromuscular control after unilateral ACLR across four leading sports rehabilitation centers. Approach: Test 132 patients (16?25 yrs old) longitudinally after unilateral ACLR (i) during rehabilitation and (ii) after achieving RTS clearance. Compare their performance to sex- and skill-dependent normative ranges of leg dexterity measured in 426 (and counting) control subjects matched for age. Aim 1: Asses clinical utility by quantifying the ability of the leg dexterity test to identify altered neuromuscular control of the affected leg after ACLR, and its recovery over time. Hypothesis A: Construct validity: neuromuscular control (as per the leg dexterity test) approaches normative ranges over time. Hypothesis B: Criterion (Concurrent) validity: Current RTS criteria do not fully consider neuromuscular control. The leg dexterity test will identify false negative results in those cleared for RTS (i.e., prematurely cleared for RTS as they do not meet their sex- and skill-specific normative levels of leg dexterity). Aim 2: Test clinical feasibility (is it practical?) and utility (does it provide actionable information?) of using the leg dexterity test on patients in the outpatient clinic. Hypothesis C: The System Usability Scale and a Delphi Panel will show that clinicians find the leg dexterity test clinically feasible and useful to (i) improve RTS decision-making and (ii) avoid premature RTS. Deliverables: This short Phase I project will demonstrate that the leg dexterity test has clinical and scientific value to inform RTS decisions in the outpatient clinical setting. It will also enable the commercialization goals of Phase II by: (i) obtaining feedback from potential end-users to refine the hardware?s design and portability, (ii) custom- izing software and testing protocols for clinical use, (iii) scaling-up our Cloud Analytics platform, (iv) generating reports for clinical reimbursement standards, (v) and identifying leg dexterity metrics that detect deficits in neu- romuscular control at the time of RTS clearance with high sensitivity and specificity. This project will yield a risk- reduced FDA Class I device technology ready for further clinical studies and commercialization.